The present invention relates to the coils of a superconducting magnet, and in particular to the provision of cooling means for superconducting coils which are not immersed in a bath of cryogen fluid.
Superconducting magnets typically include a number of superconducting coils within a bath of liquid cryogen. More recently, ‘dry’ cryostats have become available, wherein alternative methods of cooling the coils are used. This may involve refrigeration of the coils by conduction through a thermally conductive path to a refrigerator, or may involve a cooling loop. A cooling loop is typically a thermally conductive tube carrying a small quantity of liquid cryogen. Heat from cooled equipment is absorbed through the wall of the thermally conductive tube into the liquid cryogen. The cryogen may expand, or boil, setting up convection currents in the liquid cryogen of the cooling loop. These convection currents cause the cryogen to circulate around the cooling loop to a refrigerator, which re-cools the cryogen. In this way, cooling power is distributed around the equipment.
Japanese patent application JP61-271804 describes a superconducting electromagnet having coils wound onto a former. A heat-conducting material is placed between the coil and the former and/or over the outer surface of the coil. A heat-conducting support plate, a feature of conventional on-former coil winding, is connected to the heat conducting material, and the support plate is cooled by liquid helium circulation through a cooling pipe.
The arrangement provided by this prior art has, however, been found to be undesirable for at least the following reasons. The prior art of JP61-271804 relies upon the coils being wound onto a former, which retains the coils in use. The coils are thus retained on their inner dimension. It is preferred that no former be provided, such that the coils themselves may be reduced in diameter, or that the usable bore of the coil should be increased. It is accordingly desired that the coil should be supported on its outer diameter, although this may create difficulties in accurately aligning the coil. It is also desired that the coil should have the highest possible current density, that is, the greatest possible proportion of the coil's cross-sectional area should be occupied by conductor. This assists in reducing the overall dimensions of the coil, and hence the magnet. It may also assist in reducing the total amount of superconducting wire required, and so also in reducing the cost of the coil. Attention must also be paid to the control of hoop stress within the coil, and to the possibility of damaging eddy currents flowing in system components.
GB 1443780 A describes a resin impregnated coil with axial cooling ducts and braiding between layers of the coil separated by fibres coated in thermally conductive material. In adopting this approach the coil current density is diminished , therefore a larger coil would be required to maintain ampere turns for field generation.
EP 0 413 571 A1 describes free standing epoxy coils for a refrigerated (conduction cooled) MRI magnet that are located by spacers and end sleeves (making up the cartridge) in such a way as to be demountable. Copper foil loops and overwrap are provided within and on an outer surface of the coils. This provides mechanical strengthening, but appears to have no cooling function.
JP 6176349 addresses the clamping of a coil by two plates between axial faces for direct cooling, while JP 10189328 describes a similar thermally conductive coil thrust face for direct cooling.
It has become convention, in both conduction cooling and cooling loop arrangements, to cool coils through a thermally conductive former. The former may locate the coils on either their inner or outer diameter. Location of the coils on their outer diameters results in a reduced inner coil diameter, resulting in reduced requirement for superconducting wire, and a smaller, less expensive and bulky system. It is also enables the coil itself be located as close to the patient bore as possible, for the same reasons.
In solenoidal magnets constructed with an external former, the coils of superconducting wire themselves are impregnated with resin, to form a solid coil structure. These coils are retained on their outer and side surfaces to provide the desired solenoidal arrangement of coils. In order to achieve the correct radius of each coil, yet allow mounting of all coils to a common external former, it has been found necessary to provide a filler layer, to fill a gap between the nominal outer radius of the coil and the surface of the external former. This filler layer typically comprises the same impregnation resin as used in the coil, filled with a filler material, such as glass fibre or glass beads. The ratio of resin to filler material is selected so that the thermal contraction rate of the resulting material matches the thermal contraction rate of the impregnated coil as closely as possible.
In operation, the magnetic field generated by the solenoidal magnet will act to push each coil in one direction or the other. Typically, the external former is arranged such that a solid step is provided to prevent the coil from moving in the direction it is being pushed. The other side of the coil typically has a clamping arrangement to hold the coil in position against the step in the former.
The coils in typical dry cryostats are cooled by thermal conduction along the material in contact with or bonded to the coil, such as the external former, to a cooling means such as a cooling loop or a refrigerator. When the coils are provided with a filler layer, the cooling of the coil by thermal conduction through the external former becomes difficult. In order to extract heat from the coil through the former, the heat will need to flow through the filler layer. However, the filler layer is typically composed of materials of low thermal conductivity, such as resin and glass. The cooling of the superconducting coils is thus impeded by the presence of the filler layer.
The present invention aims to alleviate the problems of the prior art and provide equipment for effective cooling of coils having a filler layer on their outer surface, mounted on an external former.